The invention relates to a method of for making interlocked metal tube. Such tube is currently made from a long strip of a suitable metal, such as steel or stainless steel, by passing the flat strip through a succession of pairs of forming rolls which gradually form the strip into a generally S-shaped cross-section of which one or both of the ends of the "S" are open. The formed strip is bent around a mandrel into a helical formation, at the same time the downwardly-facing opened end of the "S" of the strip is forced into the bight formed by the upwardly-facing opened end of the "S" of the previously wound coil on the mandrel, the opened ends being closed sufficiently to prevent adjacent coils of the helix from becoming disengaged. The finished section of the "S" provides limited axial movement between the adjacent coils to enable the tube to be partially extended axially or to be bent into a curve.
In the process described above, the tube thus formed moves axially along the mandrel and the whole length of the formed tube rotates. Thus, there is a practical limit to the length of finished tube which can be formed by that process. Firstly, problems arise in maintaining a long length in continuous rotation. Secondly, it is very difficult after the S-shaped strip has been formed and engaged and closed with the adjacent coil to weld end-to-end lengths of such tube which are short enough to manufacture, and then to dress the weld.